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1
Have ready four 8-ounce canning jars with 2-piece lids.
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Fill a large pot with water and bring to a boil over high heat, then decrease the heat so that the water is barely bubbling.
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3
Immerse the pint jars in the canning kettle.
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4
Place the rings and lids in a separate small saucepan and cover them with hot water.
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5
Leave the jars and lids immersed while you cook the jam.
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6
If you dont have a candy thermometer to tell you when the jam is done, place a small plate in the freezer.
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7
Combine the lemon slices, water, and 1/2 cup of the sugar in a large saucepan over medium-high heat; bring to a boil and cook undisturbed until the lemon slices are translucent, about 20 minutes.
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8
Add the remaining 3/4 cup sugar and the blueberries; cook, stirring occasionally, until the mixture thickens and reaches 220F on a candy thermometer, 20 to 30 minutes.
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9
If you dont have a candy thermometer, remove the plate from the freezer and immediately spoon a teaspoon of jam onto it.
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10
Let it sit for a few minutes until the jam cools, then tilt the plate and see how much it runs.
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It will probably not wrinkle and get firm the way jams with more pectin would, but it should be thick and only slightly run.
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12
If its too runny, continue cooking it down, stirring frequently, until it thickens further.
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13
While the jam is still hot, ladle it into the hot, sterilized canning jars, leaving about 1/4 inch of head space at the top.
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14
Remove any air bubbles by running a long, nonmetallic utensil, such as a chopstick or wooden skewer, between the jar and the jam.
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15
Top with the prepared lids, close tightly, and process for 5 minutes in a hot-water bath with the jars submerged by at least 1 to 2 inches of water.
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Remove from the water and let cool, undisturbed, to room temperature.
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17
Besides making that telltale ping when it seals, the lid of a properly sealed jar should be slightly concave and not move; if the lid springs down and back when you press your finger in the center, the lid is unsealed.
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18
If it doesnt seal, process it again or store it in the refrigerator instead of at room temperature.
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19
Store sealed jars in a cool, dry place.
